Potentiometric titration process

ABSTRACT

Potentiometric titration of a wide variety of reagents in solution can be conducted using an ion selective sensing device including a standard reference electrode, a reference electrolyte and a semipermeable polymeric membrane which separates the reference electrolyte from the solution containing the ion being titrated. The semipermeable polymeric membrane contains an ester of phthalic or isophthalic acid which serves to discriminate between the ion being titrated and other ions in the medium.

United States Patent Christian [5 4] POTENTIOMETRIC TITRATION PROCESS[72] Inventor: Gary Dale Christian, Lexington, Ky. [73] Assignee: MilesLaboratories, Inc., Elkhart, Ind.

[22] Filed: Nov. 20, 1969 [21] Appl. No.: 878,312

[52] [1.8. CI ..204/l T, 204/195 [51] int. Cl ..G01n 27/40, GOln 27/46[58] Field of Search ..204/l T, 195

[56] References Cited UNITED STATES PATENTS 2,954,336 9/1960 Grutsch..204/l95 [151 3,655,526 [45] Apr. 11, 1972 OTHER PUBLICATIONS Anal.Chim. Acta," Vol. 36 1966), pp. 166 179 Primary Examiner-T. TungAttorney-Joseph C. Schwalbach, Michael A. Kondzella, Louis E. Davidsonand Harry T. Stephenson [57] ABSTRACT Potentiometric titration of a widevariety of reagents in solution can be conducted using an ion selectivesensing device including a standard reference electrode, a referenceelectrolyte and a semipermeable polymeric membrane which separates thereference electrolyte from the solution containing the ion beingtitrated. The semipermeable polymeric membrane contains an ester ofphthalic or isophthalic acid which serves to discriminate between theion being titrated and other ions in the medium.

7 Claims, 8 Drawing Figures riff/I P IIEIIIEIIIPII I I I972 3,655,526

SHEET 2 BF 3 TITRAT)ION OF Pb( WITH MEM- TITRATION OF HNo BRANE3CZONTAINING wITI-I MEMBRANE CONTAINING Dl-(Z-ETHYLHEXYL)D|(29E5THYLHEXYL) ISOPHTHALATE IQSOPHTHALATE 80 LL; 60 U 75 50 m 70 E 2IO 0 l l I I O l l I o I 2 3 4 5 o I 2 3 4 5 ml. OF KOH ml. OF NQZCZO4FIGURE FIGURE 4.

TITRATION OF KCI WITH MEMBRANE CONTAZINING Dl-(Z-ETHYLHEXYL)ISOPHTHALATE FIGURE 5.

44 INVENTOR GARY D. CHRISTIAN .ml OF AgNO PATENTED PR -I I972 3,655,526

sum 3 BF 3 ETITRATION OF HNo TITRATION OF Pb(NO WITH MEMBRANE CONTAININGWITH MEMBRANE CONTAINING DIPENTYL PHTHALATE DIPENTYL PHTHALATE so I00 74LLj-72 LLllso U Q 70 50 U7 68 40 V). 966 30 64- @20 g 62- IO I I I o 2 34 5 4 0 ml. OF KOH FIGURE 6.

TITRATION OF KCI WITH MEMBRANE CONTAINING DIFEgIBTYL PHTHALATE FIGURE 8.

I INVENTOR 4 5 I GARY D. CHRISTIAN O 3 WTZKY Mv vs. S. c. E.

POTENTIOMETRIC TITRATION PROCESS BACKGROUND OF THE INVENTION Thisinvention relates to potentiometric titrations. In one of its moreparticular aspects it relates to the detection and determination of ionsin solutions also containing other ions which ordinarily interfere withthe measurement of the particular ionto be detected by titration of thedesired ion using anion selective sensing device. For example, in thepotentiometric titration of metallic ions other metallic ions than theion being detected may be a source of interference.

Various .-methods have been used for the detection and determination ofions in solution. Among these are colorimetric test methods whichinvolve the preparation and use of chemical reagents, their addition tosolutions containing the ion in question and observation of any colorchange occurring. Some of these colorimetric methods require the use ofa spectrophotometer. Others rely upon visual colorimetric sensing andare subject to individual interpretation which can vary widely.Processes involving flame photometry or atomic absorptionspectrophotometry have also been used but these methodsare quitelaborious. A convenient technique for determining ions in solution,particularly by the method of potentiometric titration, would have wideapplicability, especially in laboratories staffed with relativelyunskilled personnel or for use in the field.

PRIOR ART .Recently analytical methods involving the use of ionselective membranes have been devised. For example U.S. Pat. No.3,398,066 to A. Ilani describes a method for determining potassium andsodium ions by means of an ion selective membrane. U.S. Pat. No.3,450,631, to R. Bloch et al. describes the separation of metal ionsusing another membrane technique. Certain selective glass electrodeshave also recently become commercially available. Such electrodescontain various special glass tips which are said to be useful formeasuring concentrations of hydrogen, silver, potassium, ammonium,sodium and lithium ions, to mention but a few. U.S. Pat. No. 3,356,596,toG. Eisenman et al. describes such a glass electrodewhich is useful formeasuring potassium ion concentrations. U.S. Pat. No. 3,450,604, to G.Eisenman describes a sodium ion electrode.

Each of the aforesaid methods suffers from certain disadvantages. Thecolorimetric methods are generally time consuming and complicated,involving the handling of chemical reagents and generally requiring theassistance of a skilled laboratory technician. The photometric andspectrophotometric methods are likewise cumbersome as well as expensive.The various electrometric methods involve the use of electrode deviceswhich may be somewhat difficult to fabricate.

Previously available potentiometric titration devices utilizing ionselective membranes have shown only narrow utility as described inIjsseling et al., Anal. Chim. Acta 36, 166 (1966), Anal. Chim. Acta 40,421 (1968), Anal. Chim. Acta 43, 77 (1968) and Anal. Chim. Acta 45, 121(1969).

SUMMARY OF THE INVENTION centration of ions to which the membrane isselectively permeable and can be used to follow the course of atitration by plotting values thereof against the volume of titrant used.The result is a conveniently obtained titration curve.

The semipermeable polymeric membrane of the ion selective sensing devicein effect constitutes an immobilized discriminating agent. The agent isconveniently immobilized and thereby made available for use in the solidphase by incorporation with a semipermeable polymericmatrix. Oneconvenient way of accomplishing such immobilization is by incorporatingthe discriminating agent into a suitable polymer and casting a membranefrom a solution of the polymer and discriminating agent in a suitablesolvent.

Materials which can be used as the polymeric matrix include ethylcellulose, polyvinyl chloride, cellulose acetate and collodion. Suchmaterials are known to be permeable to small particles, such as waterand metallic ions, and impermeable to larger molecules such as polymericspecies. Incorporated with a discriminating agent in accordance with theinstant invention the permeability becomes selective.

As discriminating agent there can be used an ester of phthalic orisophthalic acid such as dibutyl phthalate, dipentyl phthalate,di-n-octyl phthalate, dimethyoxyethyl phthalate, diphenyl phthalate ordi-( Z-ethylhexyl) isophthalate. The discriminating agent may also servethe function of plasticizer for the polymeric matrix.

In order to prepare the membranes of this invention the polymeric matrixmaterial, solvent and discriminating agent are mixed together and castupon a suitable, relatively impervious surface such as a glass, wood,plastic or metal plate. Upon drying, usually at a temperature of aboutfrom 40 to C. for a period of time of about from 5 minutes to 1 hour,the film of polymeric matrix material containing discriminating agent isavailable in the form of a polymeric membrane which can be used with apaper, glass, plastic or other suitable backing or without such backingas desired.

When the resulting membrane is incorporated into an ion selectivesensing device as described above and used as one electrode in anelectrochemical cell in which the other electrode is a standardreference electrode, the resulting system can be used inelectrochemically sensing the change in potential produced in such cellupon various chemical reactions occurring within the cell. In this waythe system can be used to follow the course of a titration and detectthe end-point thereof. Titrations of the precipitation,oxidation-reduction, compleximetric or acid-base type can thus beperformed. Titrations can be conducted in aqueous or nonaqueous systems.

DESCRIPTION OF THE DRAWING FIG. 1 is a semi-diagrammatic verticalsectional view of a concentration cell which can be used in thepotentiometric titration process of this invention. The numeral 10represents a container separated into two compartments 1] and 12 by aliquid impermeable divider 13 which is formed with at least one opening14 which is covered by a semipermeable membrane 15. Membrane 15 issecured to divider 13 in a known manner and can be a semipermeablepolymeric membrane of the type described above. It will be apparent thatcontainer 10 and divider 13 should be constructed of electricallyinsulating material. Electrodes 16 and 17 are placed on opposite sidesof the membrane in compartments 11 and 12, respectively. Electrodes 16and 17 may be any standard reference electrode, for example, saturatedcalomel or silver-silver chloride electrodes. Within compartment 12 isplaced a reference electrolyte containing a known concentration of ionsto which membrane 15 is selectively permeable. Within compartment 11 isplaced the solution in which the concentration of a particular ion is tobe determined, that is, the solution to be titrated. A means 18, such asa millivoltmeter, for measuring a difference in electrical potentialbetween electrodes 16 and 17 is connected to electrodes 16 and 17through wires 19 and 20. Buret 21 is used to titrate the solution incompartment 11.

FIG. 2 is a semi-diagrammatic vertical sectional view of a preferredexperimental set-up for conducting potentiometric titrations accordingto the process of this invention. Electrode assembly 22 comprises atubular fluid impermeable body 23 which is closed at one end by means ofa cap 24 having a central opening 25 which is covered by a semipermeablemembrane 26 which is secured to cap 24 in a known manner. A standardreference electrode 27 is maintained in position coaxially within body23 as shown by means of a mounting ring 28 positioned within body 23 ata point remote from cap 24. Electrode 27 is connected to a potentialmeasuring device 29, such as a millivoltmeter by means of wire 30. Wire31 connects the potential measuring device 29 to a standard referenceelectrode 32 which may be the same as or different from standardreference electrode 27. Electrode assembly 22 contains referenceelectrolyte 33 into which the standard reference electrode 27 isimmersed. Both electrode assembly 22 and standard reference electrode 32are immersed in an unknown solution 34 to be tested, which solution iscontained within vessel 35. Buret 36 is used to titrate the unknownsolution 34.

It is apparent that body 23, cap 24, mounting ring 28 and vessel 35should be constructed of electrically insulating material.

For example, standard reference electrode 27 can be a saturated calomelelectrode and standard reference electrode 32 a silver-silver chlorideelectrode. When reference electrolyte 33 is a 0.001 M lead nitratesolution, for example, the potential measuring device records apotential difference of 23 millivolts if the unknown solution 34contains a concentration of silver ions of 0.001 M and a potentialdifference of 59 millivolts if the unknown solution contains aconcentration of silver ions of 0.01 M. Other typical potentialresponses using a membrane containing dipentyl phthalate are shown inTable 1.

TABLE 1 Membrane Response to Various Cations Potential Cation l-=M -MAE,mV

Ag 23 59 36 H 144 261 117 Na 22 21 -l K 30 73 43 Pb" 43 79 36 Ca I2 8 Inaddition a plot of potential difference against volume of titrant addedwould result in a typical S-shaped titration curve.

EXAMPLE 1 EXAMPLE 2 The procedure of Example 1 was repeated using 0.75ml. of di-(Z-ethylhexyl) isophthalate instead of dipentyl phthalate. Theresulting membrane was colorless, translucent and stable.

EXAMPLE 3 The procedure of Example 1 was repeated except that dibutylphthalate was used in place of dipentyl phthalate.

EXAMPLE 4 The procedure of Example 1 was repeated except that di-noctylphthalate was used in place of dipentyl phthalate.

EXAMPLE 5 The procedure of Example 1 was repeated except thatdimethoxyethyl phthalate was used in place of dipentyl phthalate.

EXAMPLE 6 The procedure of Example 1 was repeated except that diphenylphthalate was used in place of dipentyl phthalate.

EXAMPLE 7 The experimental set-up of FIG. 2 was used utilizing asaturated calomel electrode for both electrodes 27 and 32, electrode 32being of the sleeve type. Solution 33 was a 0.001 M lead nitratesolution. Membrane 26 was a membrane containing di -(2-ethylhexyl)isophthalate prepared according to the procedure of Example 2. It wascircularly cut and mounted in an electrode assembly made ofpolytetrafiuoroethylene. The diameter of the exposed membrane was 8 mm.Solutions were prepared of reagent grade materials and deionizeddistilled water was used for their preparation. During titration apolytetrafluoroethylene coated magnetic bar was used to stir thesolution being tested. The potential measuring device was a Keithley6lO-B Electrometer. All cables were shielded and the cell assembly wasplaced in a copper gauze cage in order to insure stable reading andadequate grounding. In addition to the elements shown in FIG. 2 asaturated ammonium nitrate agar salt bridge was used in theelectrochemical cells. The resulting cell had the followingconfiguration:

S.C.li.; 1! MPb(NO Membrane; Sample Solution; Satd. NH NO Agar SaltBridge; Sat/d. KCI; S.C.E.

Electrode assembly 22 was stored in the open for 48 hours forpreconditioning of membrane 26. Potential readings were recorded afteran interval of l to 2 minutes. Twenty-five ml. of 0.01 M nitric acidsolution was titrated with 0.1 M potassium hydroxide solution. Theresulting titration curve was in the inverse sense to the typical welldefined S-shaped titration curve and is shown in FIG. 3.

EXAMPLE 8 Using the procedure of Example 7, 25 ml. of 0.01 M leadnitrate solution was titrated with 0.1 M sodium oxalate solution. Thetitration curve obtained is shown in FIG. 4.

EXAMPLE 9 Using the procedure of Example 7, 25 ml. of 0.01 M potassiumchloride solution was titrated with 0.1 M silver nitrate solution. Atypical well defined S-shaped curve was obtained and is shown in FIG. 5.

EXAMPLE 10 The procedure of Example 7 was repeated except that themembrane used, which contained dipentyl phthalate, was preparedaccording to the procedure of Example 1. The titration curve obtained,which was of the first derivative type, is shown in FIG. 6.

EXAMPLE 1 l The procedure of Example 8 was repeated except that themembrane used, which contained dipentyl phthalate, was preparedaccording to the procedure of Example 1. The titration curve obtained isshown in FIG. 7.

EXAMPLE 12 The procedure of Example 9 was repeated except that themembrane used, which contained dipentyl phthalate, was preparedaccording to the procedure of Example 1. The tltration curve obtained isshown In FIG. 8.

I claim:

1. In a process for conducting potentiometric titrations which comprisesadding to a solution containing a first ion, the concentration of whichis to be determined, a solution containing a second ion reactable withsaid first ion, whereby the concentration of said first ion changes in apredictable manner as said second ion is added, measuring the electricalpotential between the resulting solution and a reference electrolyte,and plotting the electrical potential throughout the course of additionof said second ion, thereby obtaining a titration curve for saidreaction, the improvement which comprises measuring said electricalpotential while said resulting solution and said reference electrolyteare in contact with opposite sides of and separated by a membranecomprising a semipermeable polymeric matrix selected from the groupconsisting of ethyl cellulose, polyvinyl chloride, cellulose acetate andcollodion, said matrix having an ester of phthalic or isophthalic acidincorporated therewith.

2. A process according to claim 1 wherein the ester is selected from thegroup consisting of dibutyl phthalate, dipentyl phthalate, di-n-octylphthalate, dimethoxyethyl phthalate, diphenyl phthalate and di-(2-ethylhexyl) isophthalate.

3. A process according to claim 1 wherein the reference electrolyte is asolution containing ions to which said membrane is selectivelypermeable.

4. A process according to claim 1 wherein the reference electrolyte is asolution containing lead ions.

5. A process according to claim 1 wherein the reference electrolyte is alead nitrate solution.

6. A process according to claim 1 wherein the titration is of anacid-base, precipitation, oxidation-reduction or complex imetricreaction.

7. A process according to claim 1 wherein the titration is conducted ina non-aqueous medium.

2. A process according to claim 1 wherein the ester is selected from thegroup consisting of dibutyl phthalate, dipentyl phthalate, di-n-octylphthalate, dimethoxyethyl phthalate, diphenyl phthalate anddi-(2-ethylhexyl) isophthalate.
 3. A process according to claim 1wherein the reference electrolyte is a solution containing ions to whichsaid membrane is selectively permeable.
 4. A process according to claim1 wherein the reference electrolyte is a solution containing lead ions.5. A process according to claim 1 wherein the reference electrolyte is alead nitrate solution.
 6. A process according to claim 1 wherein thetitration is of an acid-base, precipitation, oxidation-reduction orcompleximetric reaction.
 7. A process according to claim 1 wherein thetitration is conducted in a non-aqueous medium.